Optical transmission systems are used in various communication applications. For example, telecommunication systems can utilize optical fiber technology to transmit voice and data signals over long distances. Similarly, cable television networks transmit both analog and digital signals with optical fiber technology. In order to transmit a signal (e.g., a data signal) over an optical fiber, a light beam (“carrier”) is modulated with the information signal. The modulated carrier is then transmitted to a receiver via the optical fiber.
Coherent optical communication systems utilize the phase of the optical carrier to significantly increase the capacity and distance of optical communication. In order to extract the phase information in the receiver end, the light source has to have a very narrow spectral linewidth, i.e., high frequency purity. Considerable efforts have been made to provide laser light sources that operate with a narrow spectral linewidth. To that end, a number of methods have been developed to measure the linewidth of the laser beam to determine suitability of the laser in the coherent optical communication.
Coherent optical transmission is sensitive to the linewidth of the laser beam, which is subject to noise including a white frequency modulation (FM) noise and a low frequency (LF) FM noise. The white FM noise is constant level over the entire frequency range. The LF FM noise has a higher noise level when the frequency becomes lower, which is also called the 1/f noise. However, only the white FM noise affects the quality of the coherent optical transmission, while the LF FM noise can be cancelled by the receiver of the optical signal, or does not affect the receiver performance. Accordingly, there is a need to measure the linewidth of the optical signal, and components of the noise of the measurement to understand the suitability of the laser for coherent optical communication.
For example, the method described in T. Okoshi et al., “Novel method for high resolution measurement of laser output spectrum,” Electronics Letters, vol. 16, p. 630 (1980), measured the linewidth using a simple delayed self-heterodyne method, but does not separate the white FM noise and the LF FM noise.
Another method described in Y. Yamamoto et al, “Quantum phase noise and linewidth of a semiconductor laser,” Electronics Letters, vol. 17, p. 327 (1981), uses an optical frequency discriminator to convert the FM noise into AM noise, thus enabling the direct measurement of the FM noise spectrum. However, the frequency discriminator requires careful calibration and feedback for laser wavelength stabilization, which is complicated and difficult.
The method described in K. Kikuchi, “Characterization of semiconductor-laser phase noise and estimation of bit-error rate performance with low-speed offline digital coherent receivers”, Optics Express, vol. 20, p. 5291 (2012) and K. Matsuda et al, “A Study of Laser While and Brownian FM noise in Coherent QPSK Signals,” Conference on Optical Fiber Communications (OFC), paper W4K.4 (2014) separates the LF noise using coherent detection with a reference to very narrow linewidth laser with a very similar wavelength to the laser under test. Therefore, for different lasers different reference has to be used, which is undesirable.
Accordingly, there is a need for a system and method that can separate or measure the white and LF components of the FM noise in a configuration that does not require a reference laser or laser wavelength stabilizer and frequency discriminator.